The Fates of Merging Supermassive Black Holes and a Proposal for a New Class of X-Ray Sources

We perform N-body simulations on some of the most massive galaxies extracted from a cosmological simulation of hierarchical structure formation with total masses in the range ${10}^{12}{M}_{\odot }<M_{tot}<3\times {10}^{13}{M}_{\odot }$ from $4\ge z\ge 0$. After galactic mergers, we track the dynamical evolution of the infalling black holes (BHs) around their host’s central BHs. From 11 different simulations, we find that, of the 86 infalling BHs with masses > ${10}^4{M}_{\odot }$, 36 merge with their host’s central BH, 13 are ejected from their host galaxy, and 37 are still orbiting at $z=0$. Across all galaxies, 33 BHs are kicked to a higher orbit after close interactions with the central BH binary or multiple, after which only one of them merged with their hosts. These orbiting BHs should be detectable by their anomalous (not Low Mass X-ray Binary) spectra. The X-ray luminosities of the orbiting massive BHs at z=0 are in the range ${10}^{28}-{10}^{43}$ $\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$, with a currently undetectable median value of ${10}^{33}$ $\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$. However, the most luminous $\sim$5% should be detectable by existing X-ray facilities.